FMCW radar sensor systems are believed to be used in driver assistance systems for motor vehicles, for example, for an automatic distance regulation or for early detection of the risk of a collision. The radome, which covers the antenna and is usually formed by a radar lens, is exposed to weather conditions and road dirt to a particularly high degree, so that a lossy, reflecting dielectric coating (dirt or water film) may easily be formed on the radome.
This is believed to significantly reduce the transmission and reception performance of the radar sensor, so that the locating depth and locating reliability are substantially limited and the radar sensor may even be entirely blinded. For example, at a radar frequency of 76.5 GHz, a 0.3 mm thick water film on the radome causes approximately 50% of the emitted power to be reflected on this water film and approximately 90% of the residual power is attenuated by absorption.
For purposes of traffic safety it is important that a blinding or limitation of the function of the radar sensor system can be detected as immediately as possible.
The functional principle of an FMCW radar sensor system (frequency modulated continuous wave) is that the radar signal is sent continuously; however, the frequency of this signal is modulated periodically using up and down ramps. The mixer mixes a portion of the transmission signal with the signal received from the antenna and thus produces a mixed product, the frequency of which corresponds to the difference between the frequency of the instantaneous transmission signal and the frequency of the received signal.
When a radar echo is received from a located object, the frequency of the mixed product is thus a function of the signal transit time and accordingly the distance of the object but also of the relative velocity of the reflecting object due to the Doppler Effect. Each located object therefore emerges in the spectrum formed from the time-dependent signal and the mixed product on each modulation ramp as a peak in the frequency depending on the distance and the relative velocity. By comparing the frequency positions of peaks—originating from the same object—on modulation ramps having a varying gradient, it is possible to determine the distance and the relative velocity of the object.
A reflecting coating on the radome may be seen as an “object” which has the relative velocity of zero and the distance of which corresponds to the distance between the antenna and the radome. This distance is typically of a size of approximately 2-6†cm; however, it may also be larger, for example, if the radar sensor is installed covered in the vehicle, such as behind a bumper which then forms the radome which may be susceptible to coating. However, the sensor system is generally designed for locating objects, the distance of which is between approximately 0.5 m and approximately 250†m, i.e., many times greater than the distance between the antenna and radome. When the object distance approaches zero at a relative velocity of zero, the frequency of the time-dependent signal also tends toward zero, and the period of this signal is consequently large in relation to the duration of the modulation ramp. The result of this is that it is not possible to determine such small frequencies with adequate accuracy. The peak, which would have been caused by the radome coating, is therefore outside of the evaluable range of the spectrum and can thus not be used for detecting the radome coating in conventional radar sensor systems.
A method for detecting a radome coating is discussed in EP 1 980 874 A2, which is based on the fact that the signal reflected on the radome results in a shift of the operating point of the mixer. By comparing the instantaneously measured mixer operating point with the mixer operating point in a coating-free radome, it is therefore possible to detect a radome coating.
Since, however, the mixer operating point for the coating-free radome depends on the respective installation tolerances during the installation of the radar sensor, the mixer operating point must be measured elaborately in several discrete frequencies within the allowed frequency range. Moreover, the result is falsified due to the fact that the mixer operating point is temperature-dependent and is also subject to aging influences. In order to mitigate these influences, the mixer operating point-reference curve must be constantly relearned, which limits the suitability of the radar sensor system for serial production, particularly due to the fact that it is difficult to ensure that the radome is actually coating-free when the reference curve of the mixer operating point is recorded or learned.